Some lighting equipment provides multiple colors by placing dichroic filters secured on a rotatable color wheel in the path of a light beam. Various color combinations can be produced by overlapping the filters.
The lighting fixture of U.S. Pat. No. 4,800,474 comprises two rotatable color wheels. The disclosure of this patent is herein incorporated by reference to the extent necessary for proper understanding. Each color wheel has a set of dichroic filters mounted about the periphery of a hub in which each of the filters in the set can be selectively positioned in a light beam by rotation of the color wheel. One of the color wheels is equipped with long wave pass dichroic filters while the other color wheel is equipped with short wave pass dichroic filters. By aligning various combinations of these filters, a number of different colors with different saturations can be produced.
The cutoff wavelengths for the dichroic filters are selected to be different at the long and short wavelengths for the filter set, such that there is a perceived uniform graduation of colors across the spectrum.
Each of the dichroic filters mounted on the color wheels is in a shape of a trapezoid and is mounted adjacent other filters, such that there is no blanking of light or leakage of light in the process of changing from one filter to the next.
This color wheel is constructed by joining two laminated aluminum plates. The diameter of the first plate is smaller than the diameter of the second plate. The difference in the diameters of the two plates form a step. The dichroic filters are bonded by an adhesive, preferably RTV silicon rubber, at the step. The dichroic filter is not easily removable after the filter is bonded to the plates by the adhesive.
The color wheel of U.S. Pat. No. 5,060,126 comprises a polygonal metal plate, which has slotted metal bars seated on the side edge of the polygonal plate with rectangular glass plates. One embodiment features dichroic filters adhesively held in the slots of the metal bars. The glass plates are not in direct contact with the metal bars and are spaced close to each other, but not in contact, in a peripheral array. The dichroic filters are not readily removed from the metal slots after the adhesive is applied.
The number of possible color combinations that can be produced by such a lighting fixture may be limited by the fixed number of secured filters on a particular color wheel. Some stage lighting applications may need different effects, and hence different sets of color filters during the course of a production. This may require custom color wheels.